Zeolite catalysts are employed in the fluid catalytic cracking (FCC) refining process to convert typically low value vacuum gas oils into distillates, primarily gasoline. Due to catalyst breakage and attrition during FCC conversion and regeneration, catalyst “fines” are created, which may have particle sizes of less than 10 microns in diameter, in the catalyst inventory. These particulates are very easily entrained in any gas. Because it is undesirable to permit these particulates to pass into the atmosphere in the flue gases, electrostatic precipitators (ESP) have been employed as a means of trapping such particulates before release into the atmosphere.
The most common ESP in industrial applications is a plate-wire ESP, where gas flows between positively charged metal plates and negatively charged electrode wires. A high voltage applied between the plate and wire causes an electrically charged corona to form in the gas between the plate and the wire. An alternative to the plate-wire ESP is a flat plate ESP, where corona generating wires, or discharge electrodes, are placed ahead of collection plates. During operation of the ESP, a particulate-bearing gas passes through negatively charged corona and the particulates themselves become negatively charged. The charged particulates are then carried in the flowing gas stream to positively charged collection plates that are positioned parallel to the direction of the gas flow. The particulates accumulate on the collection plates and are removed by various techniques for disposal.
One problem associated with electrostatic precipitators is the re-entrainment of particulates in the flue gas when they are dislodged from collecting plates, typically by the application of a mechanical displacing force, or “rapping.” The particle re-entrainment during rapping, often referred to as “rapping puff” accounts for a majority of particulate matters (PM) escaping the ESP with the flue gas. It has been proposed that, in order to minimize re-entrainment, the collecting surface should be struck by a force of proper intensity to snap the dust cake formed on the collecting electrode loose and allow it to slide down in cake form into a dust hopper from which it can be collected. Methods have also been proposed to further minimize re-entrainment by adding baffles to collecting electrode plates or by employing jets of secondary gas (see, for example, U.S. Pat. No. 3,988,130) in order to keep the bulk of the flue gas away from the dust cake, thereby providing a quiescent zone for dust to slide downward during rapping. It has also been proposed to stop or reverse gas flow through a portion of a precipitator during rapping (see, for example, U.S. Pat. No. 3,900,299). However, these approaches will not be effective for a large industry ESP because of long settling time required for fine particles with a low terminal velocity.
As finer and finer particulates are being regulated (EPA PM10 and PM2.5 regulations), re-entrainment of micron and submicron sized particulates is becoming more problematic. A need exists for novel methods for controlling fine particulate matter emissions.